The present invention relates to rubber mixtures based on uncrosslinked rubbers and crosslinked rubber particles (so-called rubber gels) and to multifunctional isocyanates based on polyuret. The rubber mixtures according to the present invention are suitable for producing rubber vulcanizates, which exhibit an advantageous combination of mechanical properties, such as modulus at 300% elongation, elongation at break, tear strength and abrasion resistance. The vulcanizates produced from the rubber mixtures according to the present invention also have a lower density and this has a positive effect on the weight of the rubber molded articles produced from the vulcanizates, in particular tires or tire parts.
It is known that rubber mixtures composed of uncrosslinked rubbers and crosslinked rubber particles (rubber gels) as fillers produce vulcanizates which have low rebound resilience at ambient temperature (good skid resistance in the wet) and high rebound resilience at 70xc2x0 C. (low rolling resistance) during vulcanization with conventional vulcanizing agents (for example sulfur vulcanization).
Reference is made by way of example in this context to U.S. Pat. No. 5,124,408, U.S. Pat. No. 5,395,891, DE-A 197 01 488, DE-A 197 01 487, DE-A 199 29 347, DE-A 199 39 865, DE-A 199 42 620.
The reinforcing effect of the microgels in vulcanizates (modulus at 300% elongation xe2x80x94S300-, elongation at break xe2x80x94D-, tear strength and abrasion) can be improved for industrial use. This is demonstrated, in particular, by the fact that large quantities of gel must be used to adjust industrially relevant S300 values.
Overfilling of the mixtures results from these large quantities of gel and consequently the tear strengths and the elongations at break of the vulcanizates decrease. The object is therefore to find measures to increase the modulus of lowfilled gel-containing rubber vulcanizates. The object is also to reduce the DIN abrasion.
It is also known to vulcanize natural rubber with carbon black as filler with diisocyanates. The vulcanizates obtained in this way do not, however, have satisfactory mechanical properties. In addition, the vulcanizates adhere very strongly to the metal parts of the vulcanizing molds used (O. Bayer, Angewandte Chemie, edition A, year 59, No. 9, p. 257-288, September 1947).
According to the present invention, rubber mixtures will now be provided which allow production of vulcanizates with improved mechanical properties (product of modulus at 300% elongation and elongation at break) and a lower vulcanizate density, and this is desirable, for example, in tires or individual tire components.
The present invention, therefore, provides rubber mixtures containing uncrosslinked, double bond-containing rubbers (A), crosslinked rubber particles (B) and multifunctional isocyanates based on biuret (C), wherein, based on 100 parts by weight (phr) of the rubber components (A) respectively, the component (B) content in the mixture is 1 to 150 parts by weight and the multifunctional isocyanate content based on biuret (component C) is 1 to 100 parts by weight.
Preferred rubber mixtures according to the present invention are those which, based on 100 parts by weight of the rubber component (A) respectively, have 5 to 100 parts by weight of crosslinked rubber particles (component B) and 3 to 50 parts by weight of multifunctional isocyanates based on biuret (component C).
Double bond-containing rubbers are taken to mean those rubbers which are designated R-rubbers in accordance with DIN/ISO 1629. These rubbers have a double bond in the main chain. They include, for example:
Double bond-containing rubbers should also, however, be taken to mean those rubbers which are called M-rubbers in accordance with DIN/ISO 1629 and, in addition to the saturated main chain, have double bonds in the side chain. These include, for example, EPDM.
The double bond-containing rubbers of the above-mentioned type to be used in the rubber mixtures according to the present invention can, of course, be modified by functional groups which can react with the functional isocyanates based on polyuret to be used and, as will be described below, can improve a coupling of the crosslinked rubber particles to the surrounding rubber matrix in the vulcanized state.
Uncrosslinked rubbers which are functionalized by hydroxyl, carboxyl, amino and/or amide groups are particularly preferred. The functional groups can be introduced directly during polymerization, by copolymerization with suitable comonomers, or after polymerization, by polymer modification.
The introduction of such functional groups by polymer modification is known and described for example in M. L. Hallensleben xe2x80x9cChemisch modifizierte Polymerexe2x80x9d in Houben-Weyl Methoden der Organischen Chemie, 4th Edition, xe2x80x9cMakromolekulare Stoffexe2x80x9d, part 1 to 3; Georg Thieme Verlag Stuttgart, New York, 1987; pages 1994 to 2042, DE-A 2 653 144, EP-A 464 478, EP-A 806 452 and German Patent Application No. 198 32 459.6.
The quantity of functional groups in the rubbers is conventionally 0.05 to 25 wt. %, preferably 0.1 to 10 wt. %.
Crosslinked rubber particles known as rubber gels, in particular those obtained by appropriate crosslinking of the following rubbers:
are used in the mixtures according to the present invention as crosslinked rubber particles.
The rubber particles to be used according to the present invention conventionally have particle diameters of 5 to 1,000 nm, preferably 10 to 600 nm (diameters to DIN 53 206). They are insoluble owing to crosslinking and may be swollen in solvents, for example toluene. The swelling indices of the rubber particles (Qi) in toluene are approximately 1 to 15, preferably 1 to 10. The swelling index is calculated from the weight of the solvent-containing gel (after centrifugation at 20,000 rpm) and the weight of the dry gel, wherein Qi=wet weight of the gel/dry weight of the gel. The gel content of the rubber particles according to the present invention is conventionally 80 to 100 wt. %, preferably 90 to 100 wt. %.
Production of the crosslinked rubber particles (rubber gels) to be used from the basic rubbers of the above-mentioned type, is known in principle and described, for example, in U.S. Pat. No. 5 395 891 and EP-A 98 100 049.0.
In addition it is possible to increase the particle sizes of the rubber particles by agglomeration. Production of silica/rubber hybrid gels by coagglomeration is also described, for example, in the German Patent Application No. 199 39 865.8.
Of course, like the above-mentioned uncrosslinked double bond-containing rubbers, the crosslinked rubber particles can also be modified by suitable functional groups which, as mentioned above, can react with the multifunctional isocyanates to be used and/or bring about an improvement in the coupling of the rubber particles to the surrounding rubber matrix in the vulcanized state.
Preferred functional groups are again the hydroxyl, carboxyl, amino and/or amide groups. The proportion of these functional groups corresponds to the proportion of these groups in the above-mentioned, uncrosslinked, double bond-containing rubbers.
Modification of the crosslinked rubber particles (rubber gels) and introduction of the above-mentioned functional groups is also known to the person skilled in the art and described, for example, in the German Patent Applications Nos. 199 19 459.9, 199 29 347.3, 198 34 804.5.
Only modification of the corresponding rubbers in aqueous dispersion with appropriate polar monomers which can introduce a hydroxyl, amino, amide and/or a carboxyl group into the rubbers needs to be mentioned at this point.
Modified crosslinked rubber particles which are modified on the surface by xe2x80x94OH; xe2x80x94COOH; xe2x80x94NH2; xe2x80x94CONH2; xe2x80x94CONHR-groups and which are in the above-mentioned quantity range are particularly preferably used in the rubber mixtures according to the present invention.
The multifunctional isocyanates based on polyuret (component C), which have at least two isocyanate groups in the molecule, are derived from the following idealized basic structure (DE-A 11 01 394; G. W. Becker, D. Braun, Kunststoffhandbuch, Vol. 7, Hanser-Verlag; Chapter 10.1 by G. Mennicken and W. Wieczorrek, xe2x80x9cLacke, Anstrichmittel und Beschichtungenxe2x80x9d, page 540 ff, Laas et al J. prakt. Chem. 336 (1994) 185). 
The degree of oligomerization n is 1 to 25, preferably 1 to 10. Q1, Q2 and Q3 are aliphatic, cycloaliphatic and aromatic bridges such as hexamethylene, toluylene, diphenylmethylene, naphthylene bridges. Q1, Q2, Q3 are derived from the known aliphatic, cycloaliphatic and the aromatic diisocyanates. Hexamethylenediisocyanate (HDI), for example, is, a representative of the aliphatic, multifunctional isocyanates. 1-isocyanato-3-(isocyanatomethyl)-3,5,5-trimethyl-cyclohexane (isophorondiisocyanate/IPDI) for example, is, a representative of the cycloaliphatic multifunctional isocyanates. Representatives of the aromatic multifunctional isocyanates include: 2,4- and 2,6-diisocynatotoluene and the corresponding industrial isomer mixture (TDI); diphenylmethanediisocyanates, such as diphenylmethane-4,4xe2x80x2-diisocyanate, diphenylmethane-2,4xe2x80x2-diisocyanate and diphenylmethane-2,2xe2x80x2-diisocyanate and the corresponding industrial isomer mixtures (MDI). Naphthalene-1,5-diisocyanate (NDI) and 4,4xe2x80x2,4xe2x80x3-triisocyanatotriphenylmethane should also be mentioned.
To avoid a premature crosslinking reaction, for example during compounding (reduction of the susceptibility to scorching of the mixtures), it may be necessary to use the isocyanate groups in blocked form, temperature-reversible blocking (masking) of the isocyanate groups with special alcohols, phenols, caprolactams, oximes or xcex2-dicarbonyl compounds being advantageous.
Multifunctional isocyanates based on polyuret which derive from hexamethylenediisocyanate (HDI) are particularly preferred: 
Products with such a structure are sold, for example, by Bayer AG under the names Desmodur(copyright) N 100 and Desmodur(copyright) N 3200.
The rubber mixtures according to the present invention can contain further known rubber auxiliary agents and fillers. Particularly suitable fillers for producing the rubber mixtures and vulcanizates according to the present invention are, for example:
carbon blacks. The carbon blacks to be used in this case are produced by the lamp black, furnace or gas black process and have BET surface areas of 20 to 200 m2/g, such as SAF-, ISAF-, IISAF-, HAF-, FEF- or GPF-carbon blacks.
highly dispersed silica, produced, for example, by precipitation of solutions of silicates or flame hydrolysis of silicon halides with specific surface areas of 5 to 1,000, preferably 20 to 400 m2/g (BET surface area) and primary particle sizes of 5 to 400 nm. The silicas can optionally also be present as mixed oxides with other metal oxides, such as Al, Mg, Ca, Ba, Zn and Ti oxides.
synthetic silicates, such as aluminum silicate, alkaline earth silicate, such as magnesium silicate or calcium silicate with BET surface areas of 20 to 400 m2/g and primary particle diameters of 5 to 400 nm.
natural silicates, such as kaolin and other naturally occurring silicas.
metal oxides, such as zinc oxide, calcium oxide, magnesium oxide, aluminum oxide.
metal carbonates, such as calcium carbonate, magnesium carbonate, zinc carbonate.
metal sulfates, such as calcium sulfate, barium sulfate.
metal hydroxides, such as aluminum hydroxide and magnesium hydroxide.
glass fibers and glass fiber products (laths, strands or glass microbeads).
thermoplastic fibers (polyamide, polyester, aramide).
The fillers can be used in quantities of 0.1 to 100 parts by weight, based on 100 parts by weight, of the rubber component A.
The above-mentioned fillers can be used alone or in a mixture with one another.
Rubber mixtures which contain 10 to 100 parts by weight of crosslinked rubber particles (component B), 0.1 to 100 parts by weight of carbon black and/or 0.1 to 100 parts by weight of so-called light fillers of the above-mentioned type, based on 100 parts by weight of the rubber component A respectively, are more preferred. The quantity of fillers when using a mixture of carbon black and light fillers is approximately 100 parts by weight maximum.
The rubber mixtures according to the present invention can, as mentioned, contain further rubber auxiliary agents, such as crosslinking agents, vulcanization accelerators, antioxidants, heat stabilizers, light stabilizers, anti-ozonants, processing aids, plasticizers, tackifiers, blowing agents, dyes, pigments, wax, extenders, organic acids, retarders, metal oxides and filler activators, such as triethanolamine, polyethyleneglycol, hexanetriol, bis-(triethoxysilylpropyl)-tetrasulfide. The rubber auxiliary agents are described, for example, in J. van Alphen, W. J. K. Schxc3x6nbau, M. van Tempel Gummichemikalien, Berliner Union GmbH, Stuttgart, 1956 and in the Handbuch fxc3xcr die Gummiindustrie, Bayer AG, 2nd Edition, 1991.
The rubber auxiliary agents are used in conventional quantities which depend, inter alia, on the application. Conventional quantities are, for example, 0.1 to 50 parts by weight, based on 100 parts by weight of rubber (A).
The rubber mixtures according to the present invention can also contain further conventional crosslinking agents such as sulfur, sulfur donors, peroxides or other crosslinking agents, such as diisopropenylbenzene, divinylbenzene, divinylether, dinvinylsulfone, diallylphthalate, triallylcyanurate, triallylisocyanurate, 1,2-polybutadiene, N,Nxe2x80x2-m-phenylenemaleimide and/or trallyltrimellitate. The acrylates and methacrylates of polyhydric, preferably dihydric to tetrahydric C2- to C10-alcohols, such as ethylene glycol, propanediol-1,2-butanediol, hexanediol, polyethyleneglycol with 2 to 20, preferably 2 to 8 oxyethylene units, neopentylglycol, bisphenol A, glycerol, trimethylpropane, pentaerythritol, sorbitol with unsaturated polyesters of aliphatic diols and polyols and maleic acid, fumaric acid and/or itaconic acid can also be considered.
Sulfur and sulfur donors in the known quantities, for example in quantities of 0.1 to 10, preferably 0.5 to 5, based on 100 parts by weight of rubber component (A) are preferably used as crosslinking agent.
The rubber mixtures according to the present invention can also contain vulcanization accelerators of the known type, such as mercaptobenzothiazoles, mercaptosulfenamides, guanidines, thiurames, dithiocarbamates, thioureas, thiocarbonates and/or dithiophosphates. Like the crosslinking agents, the vulcanization accelerators are used in quantities of approximately 0.1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of rubber component (A).
The rubber mixtures according to the present invention can be produced in a known manner, for example by mixing the solid individual components in the units, such as rollers, closed mixers or mixing extruders, suited thereto. The individual components are conventionally mixed with one another at mixing temperatures of 20 to 100xc2x0 C.
The rubber mixtures according to the present invention can also be produced in that, from the latices of rubber component (A), the component (B) in latex form and the other components are mixed into the latex mixture (components A+B) and subsequently worked up by conventional operations, such as evaporation, precipitation or freezing coagulation.
The primary goal, when producing the rubber mixture according to the present invention, is the mixing components being intimately mixed together and that a good dispersion of the fillers used is achieved in the rubber matrix.
The rubber mixtures according to the present invention are suitable for producing rubber vulcanizates by appropriate crosslinking reactions with the known crosslinking agents and are used to produce molded articles of all kinds, in particular to produce cable sheaths, hoses, driving belts, conveyor belts, roller coverings, tire components, shoe soles, ring seals, damping elements and diaphragms.